Conventionally, aluminum that is superior in heat conductance is used in a heat exchanger. In a technology of chemical conversion coating for such heat exchanger, a workpiece is immersed in a liquid for chemical conversion coating which includes a low pH acid or high pH base in addition to fluorine or nitrogen. Accordingly, a rust preventive film is formed, and thus a large volume of wastewater has been discharged. For example, in a treatment technology for a heat exchanger which is described in Patent Document 1, aluminum is processed by a boehmite treatment so that a rust preventiveness becomes higher, and a catalyst metal is supported by the aluminum.
In the above-mentioned technology described in Patent Document 1, the workpiece at ordinary temperature needs to be heated again, for the boehmite treatment, at a hot water treatment process or a steam contacting process. Moreover, in a catalyst attachment process, the workpiece is required to be immersed in a liquid containing a catalyst and be dried. A surface treatment process is needed for attaching a rust resistant metal to the workpiece. Those processes may have been unsuitable for a consecutive production line from an assembly step and a brazing step of the heat exchanger. Moreover, the surface treatment process is essential, because the aluminum may be corroded if the surface treatment is skipped.